This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 10-206605, filed Jul. 22, 1998, the entire contents of which are incorporated herein by reference.
The present invention relates to an improvement in the quality of a semiconductor thin film and an improvement in the performance of a semiconductor thin film device, and is widely applicable to functional thin film devices such as thin film transistors and photovoltaic devices.
It is widely known that the electrical and photoelectrical properties of amorphous Group IV semiconductors are remarkably improved by terminating unjoined bonds (dangling bonds) of, for example, an amorphous semiconductor or polycrystalline semiconductor, with hydrogen atoms to inactivate them. For introducing hydrogen atoms, techniques of implanting activated hydrogen gas into a semiconductor material that does not contain hydrogen or that has insufficient termination with hydrogen are mainly used. Specific examples include an ion implanting method and a plasma hydrogen doping method. However, these methods introduce hydrogen particles of high energy and thus cause damages to the semiconductor bulk material, or require a high-temperature process at a certain temperature level or higher to incorporate hydrogen atoms efficiently. Further, they require both steps of forming a semiconductor material and of hydrogenating it, thus increasing the number of processing steps.
On the other hand, with chemical vapor deposition methods including plasma CVD, in which a semiconductor thin film is formed by decomposing a raw material gas containing hydrogen atoms, such as a silane-based gas, hydrogen atoms are incorporated within a semiconductor film already immediately after the deposition. Thus, the methods do not require a process of introducing hydrogen in a later stage, making it possible to form a hydrogenated semiconductor thin film in a simple manner. In addition, since they can form a high-quality thin film on an inexpensive substrate by the low-temperature process, it is expected that the cost for the device can be reduced, and at the same time, the performance of the device can be improved. Actually, these methods can produce amorphous silicon, for example, with a quality applicable to functional devices such as photovoltaic devices and thin film transistors.
However, when a hydrogenated semiconductor thin film is deposited by the plasma CVD method, it is not easy to control the process so as to incorporate a necessary and sufficient amount of hydrogen for terminating the dangling bonds. In practice, an excessive amount of hydrogen is present in the film, resulting in instability of the film and non-uniformity of the micro-structure of the film. Further, even if hydrogen is incorporated efficiently in terms of amount, the bonding strength, that is, the bonding energy, with a Group IV semiconductor atom, is not constant, but weak bonds and strong bonds exist dispersively, which causes a turbulence of the film structure and influences the stability, ultimately deciding the electrical properties of the semiconductor material. Indeed, in a hydrogenated amorphous semiconductor or the like, when the process of releasing hydrogen contained in the film is analyzed as the temperature is increased while heating the film, and the temperature profile of its hydrogen releasing amount is observed, a broad releasing process can be observed in a broad range from some tens of degrees (xc2x0 C.) or even 100 degrees or higher. The correlation between released hydrogen and temperature corresponds to the bonding energy between the Group IV semiconductor and hydrogen atom within the film, and it implies that the dispersion is large. This is also the case for hydrogenated semiconductor films which contain crystalline materials.
In view of the above-described problems of the conventional techniques, an object of the present invention is to improve the quality of a hydrogenated semiconductor thin film formed by a low-temperature plasma CVD method by controlling the state of hydrogen atoms in the thin film, as well as to enhance the performance of the semiconductor thin film device.
The present invention provides a semiconductor thin film which is deposited by using a chemical vapor deposition method at an underlying layer temperature of 400xc2x0 C. or less, and contains, as main component elements, a Group IV atom and hydrogen atom, wherein a temperature dependency of an amount of release of hydrogen atoms within the film when the film is heated from room temperature exhibits a profile having a peak of the hydrogen releasing amount at 370xc2x0 C. or higher and 410xc2x0 C. or less, and a half-value width of the peak is 30xc2x0 C. or less, more preferably 20xc2x0 C. or less.
Further, the present invention provides a thin film device comprising a semiconductor unit portion including a semiconductor thin film of the present invention and an electrode portion including a electrically conductive thin film, wherein these portions are formed on the same substrate.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.